Update `ping_timeout` docs to accord with engineio behavior

See:
5c996bec85/src/engineio/async_server.py (L30-L32)
5c996bec85/src/engineio/server.py (L31-L33)

Those docstrings correctly reflect the defaults, which are defined here: 5c996bec85/src/engineio/base_server.py (L22)

src/socketio/async_server.py

@@ -64,7 +64,7 @@ class AsyncServer(base_server.BaseServer):
                           is a grace period added by the server.
     :param ping_timeout: The time in seconds that the client waits for the
                          server to respond before disconnecting. The default
-                         is 5 seconds.
+                         is 20 seconds.
     :param max_http_buffer_size: The maximum size of a message when using the
                                  polling transport. The default is 1,000,000
                                  bytes.

src/socketio/server.py

@@ -73,7 +73,7 @@ class Server(base_server.BaseServer):
                           is a grace period added by the server.
     :param ping_timeout: The time in seconds that the client waits for the
                          server to respond before disconnecting. The default
-                         is 5 seconds.
+                         is 20 seconds.
     :param max_http_buffer_size: The maximum size of a message when using the
                                  polling transport. The default is 1,000,000
                                  bytes.